So far, the heat-sensitive recording sheets including a sheet substrate and a heat-sensitive, color-forming layer contained in the surface thereof, said color-forming layer being mainly composed of a colorless or light-colored leucodye and an acidic compound for allowing said leucodye to develop color by heating, have widely been used for facsimiles, and printers for electronic computers and measuring instruments, and attempts have been made to produce heat-sensitive, color-forming adhesive labels based on said recording sheets. Since the printing system used relies upon a thermal head, such adhesive labels are advantageous in that clear printed images of an excellent resolution are easily obtained, compared with the those in conventional impact printing systems using ink or ink ribbons. However, this type of heat-sensitive recording paper offers a problem that, when the plasticizer contained in stretch wrapping films penetrates into the heat-sensitive, color-forming layer, the developed color. It is thus impossible to apply that recording paper as a label to be used for the stretch wrapping film-packaged. In the heat-sensitive, color-forming adhesive labels for the stretch wrapping film-packaged articles, therefore, proposals have been made to provide a protective layer on the heat-sensitive, color-forming layer thereof to prevent penetration of the plasticizer into that layer (Japanese Patent Laid-Open Publication Nos. 54-128347 and 54-3594). Furthermore, it has been proposed to use as the main ingredient of the aforesaid protective layer a water-soluble polymer substance such as, for instance, polyvinyl alcohol or its modified products and starch or its modified products (Japanese Patent Laid-Open Publication Nos. 56-126193 and 56-139993).
With the heat-sensitive, color-forming adhesive labels including a protective layer composed mainly of such a water-soluble polymer substance, however, the thickness of that protective layer should be increased, since, by using only a water-soluble polymeric substance, it is difficult to sufficiently prevent penetration of plasticizers into the heat-sensitive, color-forming layer thereof. In addition, when being developed by a thermal head, there occurs certain problems caused by the thicker protective layer. For instance lowering of heat sensitivity, residue build-up and sticking. Thus, there is still left a great deal of performance improvement for practical use.